Bearings, particularly bearings for internal combustion engines, normally comprise a steel supporting strip on which is aggregated an antifriction layer constituted by a copper-lead-tin-base alloy commonly named "bronze" alloy.
In the known conventional manufacture of such bearings, the Cu-Pb-Sn alloy is applied and fixed on a steel supporting strip through two distinct techniques, namely, through the casting process or through the powder metallurgy process.
In the casting technique, the steel strip has its edges folded in the shape of a continuous canal, is heated at about 1,100.degree. C. in a reducing atmosphere, passes through a melting box where the alloy, melted at a temperature of 1,200.degree. C., is poured onto the steel strip. The strip thus lined is abruptly cooled to obtain a structure in which the lead becomes homogeneous and thinly distributed.
In the known powder metallurgy technique, which is the more flexible of the processes, it is possible to obtain strips lined with a Cu-Pb-Sn-base alloy in an ample range of compositions.
In the first stage--that of powdering--the established composition alloy is melted and, upon being poured through a small orifice, the falling metal stream is powdered by means of jets of gas or water, thereby obtaining metal powder particles, the approximately spherical shape of which can be controlled.
During the rapid cooling of each individual particle, a very thin distribution of lead is obtained through a Cu-Sn matrix. In the second stage, that of sintering, the bronze powder is deposited on a steel strip to an adequate thickness. The steel strip thus coated passes through a furnace provided with a controlled atmosphere, at temperatures between 705.degree. and 1,000.degree. C., and the powder particles sinter between themselves and on the steel supporting strip. In this stage, owing to thermodynamic phenomena, there is a rearrangement and a redistribution of the bronze phase and of the lead. This results in a certain distribution of the Pb phase which is not as thin as that of the particles in the original state in which they were powdered. Subsequently to the first sintering, the strip passes through rolling mill rolls, for the purpose of providing densification of the sintered particles and the total elimination of the porosities. Afterwards, the strip is sintered over again, so as to bring about the structural strengthening of the bronze which had undergone deformation during the previous step. The strip can then be rolled again for dimensional adjustment and/or increase of certain physicomechanical properties.
As a function of the redistribution of the Pb phase after the sintering stages, the antifriction layer obtained has a structure in which the Pb globules (or islands) are present in sizes which facilitate the continuity, and there may be an interconnection of the Pb globules, such as is illustrated in FIG. 1.
The perfecting of the typical sintered structures (FIG. 1) has been the object of constant research, with a view to increasing the properties of such alloys for application in bearings, such as resistance to fatigue, increase of load capacity, allied to good resistance to corrosion, and low coefficient of attrition. There is a consensus among manufacturers of bearing materials and manufacturers of motors that the thinner and more homogeneous and discontinuous the dispersion of the Pb phase in the bronze matrix, the better the performance of the bearings when used in heavy-duty motors. Similarly, it is widely accepted that the continuity of the Pb phase in the bronze matrix makes possible, for instance, the removal of the lead from the bearing alloy, by means of the chemical action of the acidity which takes place in lubricating oils when these are used for excessively long periods and at high temperatures.